Scientist warns large water allocations could result in fewer fish

Updated
April 08, 2014 15:35:00

The Territory Government has made its first major grant of water, since giving a licence to the Country Liberals candidate Tina MacFarlane in Mataranka last year. It is allocating almost all of the water available under its draft Mataranka plan. Eighty percent of the water is being reserved for the environment, but scientists are warning even using 20 percent of water resources across the Territory could disproportionately reduce fish stocks.

ALYSSA BETTS, PRESENTER: The Territory Government has made its first major grant of water, since giving a licence to the Country Liberals candidate, Tina MacFarlane, in Mataranka last year. It is allocating almost all of the water available under its draft Mataranka plan. 80 per cent of the water is being reserved for the environment, but scientists are warning even using 20 per cent could reduce fish stocks. The Director of the Tropical Rivers and Coastal Knowledge Program, Professor Michael Douglas, spoke to Jane Bardon.

JANE BARDON: What is some of the latest science showing about the need that some fish species, prawns and cherapin have to basically travel up and down rivers to get to the sea?

PROFESSOR MICHAEL DOUGLAS: About a third of the freshwater fish that we find in our river systems have to travel between the freshwater and the saltwater to complete their life cycles. So about a third of them need to either breed in the estuary and then move upstream or they're laying eggs further upstream and those eggs and larvae have to move downstream to develop and grow, so that's a very important aspect of maintaining the free connections between the rivers and downstream into the estuaries.

JANE BARDON: Is it possible that there are different tipping points and thresholds in different rivers in the Territory, in terms of the impacts on fish stocks when you take water out?

PROFESSOR MICHAEL DOUGLAS: We have a clear understanding now that the more freshwater that flows through these river systems the greater the catch of seafood from those river systems, but what we're not so sure about is what happens if we reduce the amount of water coming through those rivers, either in the wet season or the dry season, by using that water for other purposes. One study has suggested that something like a 20 per cent reduction in wet season flows could result in a 36 per cent reduction in the size of the barramundi population in the Daly River and the equates to something like a 75 per cent decrease in the value of the recreational fishery.

JANE BARDON: The Government is saying that it's protecting the environment by reserving 80 per cent of the water and only giving out 20 per cent. Is it possible that that won't be enough to keep fish stocks at their current level?

PROFESSOR MICHAEL DOUGLAS: 80-20 on the surface of it seems like a very generous allocation and it certainly does when you compare it with river systems down south, where they're probably struggling to get 20 or 30 per cent in for the environment, and we've got it the other way round up here. But we have a much higher expectation for the quality of our river systems and for the goods and services that those river systems provide and nobody wants to see that degraded in the Northern Territory. It is possible that in some river systems every drop of water could be contributing in some way to more fish production and so in those cases if you take 20 per cent out you may end up with a 20 per cent reduction in fish stocks. In other river systems it could be that fish numbers increase with water and then something else starts to limit them and the numbers of fish plateau out with water, in those cases you could take 20 per cent out the river and it may have no measurable impact. On the other hand there could be river systems where, for example, the numbers of fish increases with flows up to a point and then all of a sudden it skyrockets. For example, once the river systems reach their bank full and then they flood over out onto the flood plains fish production really increases very rapidly. In those situations you could take out a small amount of water and have no impact but then all of a sudden if you push it too far and take too much out the fishery could collapse so we have to be very conservative and very careful that we don't push river systems beyond those thresholds.

JANE BARDON: If the Government has misjudged the size of the water reserves available because it's now modelling wetter years instead of dryer years, what's the potential impact of that?

PROFESSOR MICHAEL DOUGLAS: It's definitely advisable that we take a very precautionary approach here that we don't want to get into the situation where we have to try and claw back water after industries have been based on using a certain amount of water and we've got it wrong, but I guess the thing is if the licences are allocated for a 10-year period then we want to make sure we've got very good estimates of what the likely water availability is over that next 10 years.

JANE BARDON: Does the Government need to make the public aware that there can be a trade-off between the fish stocks that you're reserving and decisions to basically use water to grow fruit and vegetables?

PROFESSOR MICHAEL DOUGLAS: Absolutely. I think that people need to understand that this is a trade-off situation - you get nothing for nothing in this world and more water we take out for agriculture it's possible that that will have an impact on fisheries and so we are very much looking at trading off one industry against another industry.